The present invention relates to the technological field of vehicle posture control apparatuses that avoid or suppress understeering tendencies (drifting out) and oversteering tendencies (spinning) by controlling the posture of a vehicle during cornering.
Conventionally, various vehicle posture control apparatuses are known, in which, such as in JP H06-183288A or JP H07-223520A, a target yaw rate is set based on the steering wheel angle and the vehicle speed, the actual yaw rate of the vehicle is detected with a yaw rate sensor, and when the deviation between the detected actual yaw rate and the target yaw rate has at least a predetermined value, an understeer control suppressing an understeering tendency of the vehicle intervenes or an oversteer control suppressing an oversteering tendency of the vehicle intervenes.
More specifically, in such a posture control apparatus, understeer control intervenes when the target yaw rate is larger than a value obtained by adding a predetermined threshold to the actual yaw rate, whereas oversteer control intervenes when the actual yaw rate is larger than a value obtained by adding a predetermined threshold to the target yaw rate.
In this conventional posture control apparatus, too much understeer control tends to intervene when the threshold is small. When the understeer control intervenes too early, there is the problem that this intervention of control interferes with the operation of the driver, damaging the driving experience.
In order to rectify this, the threshold determining the intervention of the understeer control should be increased, so that an early intervention of the understeer control can be inhibited. However in that case, the control will intervene only when the understeering tendency of the vehicle is already too large. Moreover, when the control finally intervenes, a strong control intervenes abruptly. Therefore, even though it is possible to ensure the stability of the vehicle, in a situation in which the understeering tendency increases until the understeer control intervenes, the driver feels that the controllability worsens, leading to a sense of instability.
In posture control apparatuses in which, different from the above-described conventional posture control apparatus setting the target yaw rate based on the steering wheel angle, the target yaw rate is set based on the lateral acceleration of the vehicle, the interference of control can be avoided by setting the intervention threshold for judging about intervention of understeer control to a relatively large value. In such a posture control apparatus, the increase of the lateral acceleration is small when the steering wheel is turned in a situation in which the vehicle is traveling straight forward (steering start), and detection of this lateral acceleration is difficult, so that the understeer control hardly intervenes at all. Therefore, considering traveling along a cornering path for example, a weak understeering tendency cannot be suppressed at the beginning of the steering near the entrance into the cornering path. As a result, a small cornering radius R has to be taken near the exit of the cornering path, or a strong understeer control may intervene. As a result, the driver may feel that the controllability is poor, or may feel a sense of instability.
Also when the behavior of the vehicle has an oversteering tendency, when the posture of the vehicle does not change in response to a steering wheel operation, the driver may feel as if the vehicle tends to understeer. Also in this situation, in a posture control apparatus in which the target yaw rate is set based on the lateral acceleration, understeer control does not intervene because the behavior of the vehicle is not in an understeering tendency. As a result, the driver may feel that the controllability is poor, or may feel a sense of instability.
In view of these problems, it is an object of the present invention to ensure a high vehicle stability and to enhance the sense of stability and the controllability felt by the driver, by improving the understeer control in a posture control apparatus.
In order to achieve these objects, according to the present invention, when a vehicle has an understeering tendency, intervention of a first understeer control suppressing this understeer tendency is performed, and when the actual yaw rate does not make a predetermined change with respect to a change of a steering wheel angle, intervention of a second understeer control is performed.
More specifically, the subject matter of the present invention is a vehicle posture control apparatus provided with a control means for controlling posture of the vehicle in yawing direction by independently controlling brakes of the vehicle""s wheels.
A special feature of the invention is that the control means carries out intervention of a first understeer control suppressing the understeering tendency when the understeering tendency of the vehicle is stronger than a preset reference value, and the control means carries out intervention of a second understeer control, in which the control amount is lower than in the first understeer control, when the actual yaw rate does not make a predetermined change with respect to a change of a steering wheel angle of the vehicle.
Thus, first, when the understeering tendency of the vehicle is stronger than a preset reference value, a first understeer control intervenes to suppress this understeering tendency. This sufficiently ensures the stability of the vehicle.
On the other hand, when the actual yaw rate does not make a predetermined change with respect to a change of a steering wheel angle of the vehicle, for example when the deviation between the change rate of the steering wheel angle and the change rate of the actual yaw rate is increasing, then the understeering tendency would increase if unchecked (initial understeering state). To avoid this, when the actual yaw rate does not make a predetermined change with respect to a change of a steering wheel angle, the initial understeering state and the understeering growth (that is, the increase of the understeering tendency) are both suppressed by intervention of the second understeer control. Furthermore, also when the posture of the vehicle does not change in response to the steering wheel angle even though the behavior of the vehicle is not in an understeering tendency, or in other words, when the driver feels that there is an understeering tendency, the feeling of the driver of being in an understeering tendency can be suppressed by intervention of the second understeer control. Thus, by intervention of the second understeer control, strong understeering tendencies can be prevented, and the sense of stability felt by the driver can be improved. In addition, the posture of the vehicle is changed in the direction intended by the driver, so that the controllability felt by the driver is improved.
Moreover, in the second understeer control, the control amount is lower than in the first understeer control. Therefore, the driver will hardly feel when the second understeer control intervenes. Moreover, even though the posture of the vehicle changes slightly in response to the driver""s steering due to the second understeer control, this posture change is not very large. Therefore, the driver feels as if the behavior of the vehicle follows the operation of the steering wheel, and feels the intervention of control even less. As a result, an awkward feeling of the driver is prevented, and the driving experience is improved.
Furthermore, when for example the understeering increases even though the second understeer control has intervened, and the understeering tendency becomes stronger than a preset reference value, then the first understeer control intervenes, replacing the second understeer control. That is to say, before the first understeer control intervenes, the second understeer control intervenes, so that the first understeer control, which has a strong control amount, does not intervene abruptly. As a result, the awkward feeling of the driver is largely rectified. Also, the second understeer control intervenes previously to intervention of the first understeer control, so that the play of the brakes is eliminated (for example, leading to a state in which the brake pads adhere to the disk rotor). Therefore, the responsiveness of the first understeer control is improved. Furthermore, due to the intervention of the second understeer control, a slight force acts on the vehicle in the yawing direction. In this situation, the understeering tendency is easily suppressed by intervention of the first understeer control. Consequently, an even higher stability of the vehicle can be ensured.
Thus, in the present invention, the second understeer control is provided separately from the first understeer control, so that while ensuring a high stability of the vehicle, the sense of stability and the controllability felt by the driver are improved.
It is preferable that the second understeer control supplies a brake pressure whose upper limit is a predetermined brake pressure that is set to be lower than the maximum brake pressure that can be supplied in the first understeer control.
Thus, the control amount during the second understeer control is lower than the control amount during the first understeer control. As a result, a second understeer control is achieved whose control intervention is not noticed by the driver, even while suppressing for example a weak understeering tendency.
It is preferable that the control means carries out intervention of the second understeer control when the steering wheel is turned from an orientation in which the vehicle is moving straight forward and the actual yaw rate does not make a predetermined change with respect to the change of the steering wheel angle when turning the steering wheel.
That is to say, when a sufficient yaw rate change with respect to the steering at the initial steering of the steering wheel is not attained, for example when the steering wheel is turned in a situation in which the vehicle is traveling straight forward, then intervention of the second understeer control is carried out. Thus, the necessary yaw rate with respect to the steering angle is attained, for example at beginning steering at the entrance into a cornering path. As a result, a situation can be avoided, in which a small cornering radius R must be taken near the exit of the cornering path. Thus, the vehicle behaves as intended by the driver, and the tracing properties with respect to a target cornering trace are improved.
Also when it is difficult to determine whether the vehicle has an understeering tendency, such as at the beginning of the steering with the steering wheel, the control intervention of the second understeer control can be carried out accurately, because the intervention judgment is based on the steering wheel angle and the actual yaw rate.
The conditions for stopping the second understeer control can be set for example as follows: After intervention of the second understeer control, the control means stops the second understeer control when a deviation between the change rate of the steering wheel angle and the change of the actual yaw rate has switched to a decreasing tendency.
That the deviation between change ratio of the steering wheel angle and the change ratio of the yaw rate switches to a decreasing tendency occurs when the position of the vehicle is changing in accordance with the steering wheel operation. Consequently, when the intervention of the second understeer control is continued, the posture of the vehicle changes more than the steering wheel angle. To avoid this, the second understeer control is stopped when the change of the actual yaw rate is close to the change of the steering wheel angle. Thus, the control does not become excessive, and an optimal second understeer control can be achieved.
It is preferable that the first understeer control controls the braking of a cornering inside rear wheel, and the second understeer control controls the braking of a cornering inside front wheel.
By letting the second understeer control control the braking of a cornering inside front wheel, which is very effective to suppress an understeering tendency, understeering tendencies can be suppressed reliably and fast.
It is preferable that the control means prohibits the intervention of the second understeer control when the vehicle has an oversteering tendency that is stronger than a preset reference value.
That is to say, when the vehicle has an understeering tendency and an oversteering tendency, for example when the vehicle drifts out while spinning, then the oversteering tendency has to be controlled first. And when the vehicle has an oversteering tendency that is stronger than a preset reference value, the intervention of the second understeer control is prohibited, so that an oversteer control suppressing the oversteering tendency is performed. This improves the stability of the vehicle even more.
It is preferable that, in a vehicle posture control apparatus performing second understeer control in addition to the first understeer control in this manner, the control means judges an understeering tendency of the vehicle with regard to the first understeer control, based on a deviation between a target yaw rate set based on lateral acceleration of the vehicle and the actual yaw rate of the vehicle.
This means, at the beginning of steering, for example when the steering wheel is turned in a situation in which the vehicle is traveling straight forward, the increase of the lateral acceleration of the vehicle is small, and moreover, the lateral acceleration is difficult to detect. Therefore, it becomes difficult to intervene with the first understeer control based on the lateral acceleration. To avoid this, a second understeer control, whose intervention is determined based on the steering wheel angle and the actual yaw rate, is provided separate from the first understeer control, thus making it possible to intervene with the second understeer control early on. Thus, weak understeering tendencies as well as situations in which the driver feels an understeering tendency are accurately suppressed. As a result, the tracing properties are improved, and the sense of stability and the controllability felt by the driver are improved.